1. Field of the Invention
The present invention relates to a calender for treating both sides of a web of paper. More specifically, the present invention relates to a calender having a roller stack which has a hard top roller, a hard bottom roller, and hard and soft rollers disposed between the top roller and the bottom roller. Working nips are formed between the juncture of one hard roller and one soft roller. A changeover nip is formed between the juncture of two soft rollers. The roller stack can be loaded in the direction of the stack by a force acting on one of the ends of the stack.
2. Discussion of the Related Art
Calenders for treating both sides of a paper web are known, for example, from the advertising brochure, "The new supercalender designs", dated May 1994, from the finn of Sulzer Papertee GmbH. These supercalenders are used to glaze coated and uncoated papers, for example, printing papers or silicon base papers. Metal rollers having a smooth and hard outer surface are known as "hard" rollers, and are primarily responsible for affecting the surface structure, such as smoothness and gloss, of the treated papers. Rollers having a flexible or soft outer surface are known as "soft" rollers, and primarily provide for uniform compression of the paper. A changeover nip is necessary so that both sides of the web of paper undergo approximately the same treatment. A supercalender typically has from 12 to 16 rollers, and the changeover nip is located in the lower half of the stack. The changeover nip is disposed in the lower half of the stack to take into account the fact that the web of paper which is running through the stack, from the top towards the bottom, is smoothed to a lesser extent at the top of the stack than at the bottom of the stack because the loading, including the weights of the rollers and the parts that may be attached to them, such as protruding weights, increases towards the bottom of the stack. This increase in the load causes the line load, compressive stress, and/or a dwell time in the working nips to increase from the top towards the bottom. As a result, these supercalenders have a great overall height, and are very expensive because of the large number of rollers.
From European reference No. EP 0 230 563 A1 a device is known which has a roller stack having seven rollers. The top roller, the bottom roller and the middle roller are configured as deflection adjustment rollers. The remaining rollers are mounted on levers. Hydraulic servomotors, which act on the levers, or more specifically, on the ends of the deflection adjustment rollers, are used to raise, lower or hold the desired position of the roller surface. The servomotors relieve the remaining rollers of the effects of any protruding weights.